Axial flow gas turbine engines include a compressor, a combustor, and a turbine spaced sequentially along a longitudinal axis. An annular flow path extends axially through the compressor, combustor and turbine. The compressor includes an array of rotating blades that engage incoming working fluid to compress the working fluid. A portion of the compressed working fluid enters the combustor where it is mixed with fuel and ignited. The products of combustion or hot gases then flow through the turbine. The turbine includes alternating arrays of vanes and rotating blades. In the turbine, energy is transferred from the flowing hot gases to the turbine blades. A portion of this energy is then transferred back to the compressor section via a rotor shaft.
To optimize the efficiency of the interaction between the turbine blades and the hot gases flowing through the turbine, the hot gases are confined to an annular space by inner and outer turbine shrouds. The inner turbine shroud is typically a plurality of platforms integral to the blades. The platforms mate with platforms of adjacent blades to form an inner flow surface for the hot gases. The outer shroud is typically a ring-like assembly disposed radially outward of, but in close radial proximity to, the outer tips of the rotating blades. The outer shroud includes a plurality of arcuate segments spaced circumferentially to provide an outer flow surface for the hot gases.
Since the shroud segments are in direct contact with the hot gases, some form of cooling is required to maintain the shroud segments within acceptable temperature limits. Cooling methods have included impingement cooling, by injecting cooling fluid onto the radially outward or back side of the shroud segment, and film cooling, by forming cooling holes through the shroud segment that produce a film of cooling fluid over the flow surface of the shroud segment. The problem is made more difficult because the shroud is a non-rotating part in the engine. As a result, the shroud cannot benefit from the rotational effects that are exerted on the cooling fluid, such as occur in a rotor blade. Flow separation is a particular problem in such coolable non-rotating structures.
Although both impingement cooling and film cooling have proven adequate in most situations, advancements in gas turbine engines have resulted in higher temperature gases flowing through the turbine. This hotter working fluid has dictated the need for improved and more efficient cooling methods. One such recently developed method is disclosed in commonly assigned, pending U.S. patent application Ser. No. 07/993,862, entitled "Turbine Blade Outer Air Seal With Optimized Cooling and Method of Fabrication". This application discloses cooling channels extending laterally through the shroud segment in a counter flow array. The channels include inlets in the back side of the shroud segment, exits ejecting cooling fluid into the inter-segment gap, and a taper in the direction of flow through the channels to control the Mach number of the fluid flowing through the channels.
A limitation to all the above arrangements is the ability to provide cooling fluid to the leading edge and trailing edge regions of the shroud segment. Each shroud segment includes retaining means adjacent the leading edge and trailing edge regions to retain the shroud segment into position within the stator structure. The retaining means are typically hooks or rails that extend laterally along the edges and radially outward from the back side of the shroud segment. The hooks and rails present an obstruction to flowing cooling fluid to this region to impinge upon the back side near the edges. Although film cooling passages may be angled to direct cooling fluid partially into these regions, forming film cooling passages at angles shallow enough to provide complete coverage is impractical. Finally, the hooks and rails prevent direct injection of cooling fluid into lateral channels under the hooks and rails and would require a cavity to extend from the back side, under the hooks and rails and over the leading edge and trailing edge regions. The latter would extend the hooks and rails further outward from the shroud segments, adding weight and stiffness to the shroud segments.
One solution is to provide a serpentine channel in the shroud segment. The serpentine channel extends along at least one of the axial edges of the segment. The serpentine channel includes an inner passage, an outer passage and a duct. The outer passage is nearest the edge and is in fluid communication through a bend passage with the inner passage. The bend passage has an upstream turn portion and a downstream turn portion. Each turn portion has an outer radius and an inner radius which bound the bend passage. The duct extends to the inner passage from an opening in the back side of the segment to permit cooling fluid to flow into and through the serpentine passage.
The feature of the serpentine channel results in convective cooling of the edge of the segment. Since this region of the segment is outward of a retaining means, such as a hook or rail, the typical methods of impingement cooling and/or film cooling are not available to this region. The retaining means presents an obstruction to getting cooling fluid into this region. The duct provides means to flow cooling fluid into the serpentine channel, which flows through the serpentine channel to the edge before exiting.
The above art notwithstanding, scientists and engineers under the direction of Applicants' Assignee are working to develop efficiently cooled turbine shroud segments for gas turbine engines.